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Reinventing the Transistor

Operating transistors at half steam could help squeeze more out of batteries.

As more and more functions are crammed into cell phones and other portable electronic devices, the capacity of their batteries is being sorely tested. To meet this challenge, Finnish handset manufacturer Nokia is turning to a novel approach in chip design that could slash the energy consumption of chips tenfold.

The technique involves the operation of chip transistors at voltage levels below the thresholds they normally need to switch on and off. In effect, this permits idle transistors or those performing low-performance functions to be placed in a kind of standby mode.

“In computer design, power consumption is getting to be a major driving force,” says Jamey Hicks, director of the Nokia Research Center in Cambridge, MA. This is particularly the case with mobile or embedded devices, he says: “The limit on the size of the device gives us a limit on the total energy budget.”

To tackle the problem, Nokia researchers have teamed up Anantha Chandrakasan, director of MIT’s Microsystems Technology Laboratories, to develop low-powered devices that use subthreshold transistors. Transistors normally act like switches – they’re the fundamental components of the digital logic gates that make up silicon chips. By switching on and off, they’re able to hold a state of 1 or 0. By using operating voltages below the normal “on” threshold, transistors can still behave like switches, but their behavior is less stable, says Chandrakasan. Below the threshold, any slight variation in the input voltage can produce massive changes in the output voltage. So the challenge in developing subthreshold transistors lies in ensuring that the input voltages are consistent enough not to produce these swings.

MIT’s Chandrakasan discovered that by selectively reducing the voltages of transistors it is possible to reduce the energy consumption by between five and ten times per operation. There is a trade off, though: “The speed of the circuit reduces,” he says. “It can be a thousand times slower.”

Chandrakasan and Nokia are working on creating a video compression chip that uses this technique to save power. Digital cameras are a standard feature of modern cell phones – but consumers often don’t realize that they come at a price. “They are pretty power hungry,” says Hicks. By developing image compression chips that can consume less power, they hope to make these camera features less of a drain on a cell phone’s overall performance. “Customers all want a long battery life and more features,” says Hicks. Even bearing in mind that the display, backlight, and phone features will still consume normal amounts of power, he predicts that such a chip could double a cell phone’s battery life.

This is not the first time chips designers have looked at using transistors below their operating voltages to conserve power, says Chandrakasan. In the 1970s, the technique was used to make chips for watches. “They were doing it primarily to reduce the energy of digital circuits, where the speed of the circuit is not so important,” he says.

Now the time has come to start delving into transistor thresholds again, says Hicks. “The circuits have gotten fast enough so that even when they are operating slowly it’s fast enough to do what we want them to do.” While processing speed is still the dominant factor in chip design for desktop computers, for mobile or embedded devices the concern has shifted. “Power is the new barrier,” says Dennis Buss, vice president of Silicon Technology Development at Texas Instruments (TI) in Dallas.

One field where the new technique might have a huge impact is radio frequency identification (RFID) tags, says Buss. These tags usually have no power source of their own. Instead, they harness the radio waves of the scanning device in order to create the power to emit a radio signal. Under these constraints, every power-saving trick goes a long way, he says. TI has already worked with Chandrakasan to produce prototypes of memory chips that enable blocks of transistors to be shifted to subthreshold voltages.

Another area where power is an issue is in medical devices, says Kaushik Roy, an electrical engineer at Purdue University in West Lafayette, IN. Roy’s group was the first to revive this technique in 1999 and is now looking at using low-power, subthreshold transistors in hearing aids. The biggest challenge, he says, is overcoming the manufacturing variations, because slight variations at the input can produce dramatic differences at the output.

One unique quality of Chandrakasan’s work is that it targets an application that requires both performance and high power, says Buss. “With cell phones there are bursts of very high performance, but then very long periods of very low activity.” Using subthreshold transistors allows one to switch back and forth, he says.

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